I. Field of the Invention:
This invention relates generally to sensing circuitry for use in a dual chamber demand pacemaker, and more particularly to a method and apparatus for obviating the need for an atrial refractory period following the occurrence of ventricular depolarization by effectively reducing to an inconsequential level the R-wave contribution to the signal on the atrial sensing lead.
II. Discussion of the Prior Art:
So-called dual chamber pacemakers have been devised which closely emulate the electrical activity of the heart. In such pacemakers, means are provided for sensing both atrial and ventricular depolarization signals and for generating pacing signals for both the ventricles and the atrium. In that the energy content of the ORS complex occurring during depolarization of the ventricles is significantly higher than that of the P-wave signal, the R-wave or the ventricular pacing spike often appear as a contaminant on the atrial sensing lead. Oversensing of the QRS on the atrial pacemaker lead has been common throughout the history of pacing and, in this regard, reference is made to a publication by Goldman, et al, entitled "Permanent Transvenous Atrial Pacing" which appeared in volume 21, page 138 of the Canadian Journal of Surgery (1978) and to an editorial authored by Furman entitled "Electrical Magnetic Interference" published in Pace in 1982. While atrial bipolar leads tend to minimize oversensing by minimizing the far field QRS spacial gradient propagated to the atrium through the use of closely spaced electrodes in the atrium, nonetheless, in practically all prior art dual chamber pacers, an atrial sense refractory blanking period, under control of the ventricular/sense pace circuitry, must be employed. Such oversensing is even more of a problem when unipolar atrial leads are used in that the major portion of the QRS gradient falls between the atrial tip electrode and the indifferent plate consisting of the can of the pulse generator.
Because the atrial QRS field and the P-wave have somewhat similar amplitude and spectral distributions, it is quite difficult to separate the two by traditional linear filtering means. In a dual chamber pacemaker operating in the DDD mode, oversensing is generally dealt with by incorporating a refractory interval following a ventricular beat or pacing spike in which the atrial sense applifier is inhibited. This renders the atrial sense amplifier inactive for a significant portion of the cardiac cycle. Because, in many instances of DDD implantation, the A/V interval is dependent upon the pacer device, there can be no inherent physiologic guarantee that the P-wave will maintain any fixed temporal relationship to the QRS, or to any other inherent mark or event. Thus, the imposition of a long atrial refractory period will increase the probability of undersensing atrial activity.
It is the principal purpose of the present invention to provide a signal processing technique permitting linear sensing of the P-wave throughout the entire cardiac cycle, while diminishing or substantially eliminating QRS oversensing by suppressing the QRS residue appearing on the atrial lead of a dual chamber pacer.
Another object of the invention is to provide a method and apparatus for use in a dual chamber pacer whereby oversensing of the QRS complex on the atrial pacemaker lead is obviated without resorting to the imposition of a refractory period.